System and method for loopback and network loop detection and analysis

ABSTRACT

A method of determining the presence of a loopback in one or more networks comprises storing information related to a test instance; sending a loopback detection beacon (LPDB) containing information related to the test instance from a port on an originating device; monitoring the port for a predetermined time period to detect LPDBs arriving at the port during the predetermined time period; and determining whether a detected LPDB contains information corresponding to the stored information, to detect the presence of a loopback. The method may determine whether a detected loopback is a port loopback, a tunnel loopback or a service loopback. The stored information related to the test instance may be deleted if an LPDB arriving at the port and containing information corresponding to the stored information is not detected within the predetermined time period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/095,273, filed on Dec. 22, 2014, and U.S. Provisional PatentApplication No. 62/045,965, filed on Sep. 4, 2014, each of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present disclosure relates to loopback and network loop detection ina network.

BRIEF SUMMARY

In accordance with one embodiment, a method of determining the presenceof a loopback in one or more networks comprises storing informationrelated to a test instance; sending a loopback detection beacon (LPDB)containing information related to the test instance from a port on anoriginating device; monitoring the port for a predetermined time periodto detect LPDBs arriving at the port during the predetermined timeperiod; and determining whether a detected LPDB contains informationcorresponding to the stored information, to detect the presence of aloopback. The method may determine whether a detected loopback is a portloopback, a tunnel loopback or a service loopback. The informationrelated to the test instance in the LPDB may be in a protocol data unit(PDU). The stored information related to the test instance may bedeleted if an LPDB arriving at the port and containing informationcorresponding to the stored information is not detected within thepredetermined time period.

In one implementation, the LPDB includes a Virtual Local Area Network(VLAN) signature, and a detected loopback is determined to be a tunnelloopback if an LPDB arriving at the port includes a VLAN signature thatmatches a VLAN signature in an LPDB sent from the port. The method maydetermine whether a detected port loopback is a hard cable loopback or afacility loopback. The information in the LPDB may contain a time stamp,so that the difference between the time stamp and the arrival time of adetected LPDB can be used to determine whether a detected loopback is ahard cable loopback or a remote facility loopback. The detected loopbackcan be determined to be a hard cable loopback if the difference issmaller than a predetermined threshold, and determined to be a facilityloopback if the difference is greater than the predetermined threshold.

In another implementation, the LPDB includes a Virtual Local AreaNetwork (VLAN) signature that includes a Customer Service Specification(CSS) and a Network Service Specification (NSS), and a detected loopbackis determined to be a service loopback if (1) the CSS of anLPDB.arriving at the port matches the CSS of an LPDB sent from the port,or (2) the NSS of an LPDB.arriving at the port matches the NSS of anLPDB sent from the port.

The monitoring and determining may be implemented on the originatingdevice, or on a device or server separate from the originating device.Affected traffic may be discarded in response to the detection of aloopback. Future traffic may be blocked at the port in response to thedetection of a loopback, and the blocking may be revertive ornon-revertive. The LPDB may be restricted to one or more selectedsections of the one or more networks. The LPDB may be generated using asoftware tool, and the software tool may be used to detect loopbackbased on the LPDB and/or to configure am NLDB.

In accordance with another embodiment, a method of determining thepresence of a network loop comprises storing information related to atest instance; setting a time period related to the test instance;sending a network loop detection beacon (NLDB) containing informationrelated to the test instance from a port on an originating device;monitoring the port for a predetermined time period to detect an NLDBarriving at the port during the predetermined time period; anddetermining whether a detected NLDB contains information correspondingto the stored information, to detect the presence of a network loop. Inone implementation, the information related to the test instanceincludes a Customer Service Specification (CSS) and a Network ServiceSpecification (NSS), and the method determines whether a detected NLDBis a network loop if the CSS and NSS of an NLDB arriving at the portmatch the CSS and NSS of an NLDB sent from the port. The originatingdevice may be a network device. The NLDB may arrive at a second port onthe originating device, and be forwarded to the port from which the NLDBwas sent. An alarm may be reported if a network loop is detected. TheNLDB may be generated or configured using a software tool, which may beused to detect the network loop.

In one implementation, traffic is either discarded or blocked if anetwork loop is detected. The blocking may be revertive ornon-revertive.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the disclosure will becomeapparent upon reading the following detailed description and uponreference to the drawings.

FIG. 1A is a diagrammatic illustration of a loopback.

FIG. 1B is a diagrammatic illustration of a network loop.

FIG. 2 is a diagrammatic illustration of port loopback that is aphysical loopback.

FIG. 3 is a diagrammatic illustration of a port loopback that is alogical loopback.

FIG. 4 is a diagrammatic illustration of a tunnel loopback.

FIG. 5 is a diagrammatic illustration of an upstream service loopback.

FIG. 6 is a diagrammatic illustration of a beacon used to detect a portloopback.

FIG. 7 is a diagrammatic illustration of the use of a beacon to detect aport loopback.

FIG. 8 is a diagrammatic illustration of a beacon used to detect atunnel loopback.

FIG. 9 is a diagrammatic illustration of the use of a beacon to detect atunnel loopback.

FIG. 10 is a diagrammatic illustration of a beacon used to detect aservice loopback.

FIG. 11 is a diagrammatic illustration of the use of a beacon to detectan upstream service loopback.

FIG. 12 is a diagrammatic illustration of a beacon used to detect anetwork loop.

FIG. 13 is a diagrammatic illustration of the use of a beacon to detecta network loop.

FIG. 14 is a diagrammatic illustration of a network with multiplesections and domain levels.

While the present disclosure is susceptible to various modifications andalternative forms, specific embodiments or implementations have beenshown by way of example in the drawings and will be described in detailherein. It should be understood, however, that the disclosure is notintended to be limited to the particular forms disclosed. Rather, thedisclosure is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of an invention as defined by theappended claims.

DETAILED DESCRIPTION

While the present disclosure is susceptible to various modifications andalternative forms, specific embodiments or implementations have beenshown by way of example in the drawings and will be described in detailherein. It should be understood, however, that the disclosure is notintended to be limited to the particular forms disclosed. Rather, thedisclosure is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of an invention as defined by theappended claims.

A loopback occurs when a frame, such as an Ethernet frame, returns tothe same port of the Network Device (ND) that it departed from. A ND canbe a Network Interface Device (NID) or any switching or routing devicewith internal connectivity. The port can be a user network interface(UNI), External Network to Network Interface (ENNI) or an InternalNetwork to Network Interface (INNI). A network loop occurs when a framereturns to the same port of the ND that it departed from, via adifferent port and after being forwarded within the ND to the port thatit departed from.

Examples of a loopback and a network loop are shown in FIGS. 1A and 1B.ND 101 has ports 101-1, 101-2, 101-3 and 101-4. In FIG. 1A, loopbackoccurs when frame 111 departs from port 101-2 of ND 101 and returns toport 101-2 via network 121. A network loop occurs as shown in FIG. 1Bwhen frame 111 departs from port 101-2 of ND 101 and returns to port101-2 via network 121 by arriving at port 101-1, and being forwardedwithin ND 101 to port 101-2.

Different types of loopback are possible. These include, for example,port loopback, tunnel loopback or service loopback.

A port loopback, occurs if the frame returns to the same port as shownin, for example, FIGS. 2 and 3. One example of port loopback is physicalloopback, as shown in FIG. 2. In FIG. 2, frame 211 enters ND 201 fromport 201-1 and departs ND 201 from port 201-2. The frame then returns toport 201-2. Physical loopback results in frames being sent back withoutany change in the source address (SA) or destination address (DA) asshown in FIG. 2. In FIG. 2, SA 211-1 and DA 211-2 of frame 211 are notinterchanged. A consequence of physical loopback is that a looped frameis broadcast back into the network in the frame flooding stage of theMedia Access Control (MAC) learning. Another consequence of physicalloopback is that the source MAC address is not learned correctly, sincethe same frame enters the ND from two different ports. For example,referring to FIG. 2, the same frame 211 enters the ND from port 201-1,and then again from port 201-2.

Another type of port loopback is logical loopback, as shown in FIG. 3.In FIG. 3, frame 211 departs ND 201 from port 201-2 and returns to port201-2. However, SA 211-1 and DA 211-2 are interchanged.

Port loopback can arise because a hard loopback is set up directly to aport, otherwise known as hard cable loopback; or a facility loopback isset up either physically or logically.

FIG. 4 shows a tunnel loopback. In this case, a tunnel loopback occursin the 802.1Q standard tunnel. A tunnel loopback occurs if a departingframe containing a VLAN signature returns to the same port with the sameVLAN signature. For example, in FIG. 4, frame 311 with VLAN signature311-3 returns to port 201-2 of ND 201 with the same VLAN signature 311-3as the frame which was sent out.

FIG. 5 shows an example of an upstream service loopback. In FIG. 5, adeparting frame 411 is sent with an associated VLAN signature 411-3comprising a Customer Service Specification (CSS) 411-4 and a NetworkService Specification (NSS) 411-5. The frame 411 is sent into thenetwork from port 201-2 of ND 201, and a loopback occurs if the frame411 is returned to the port 201-2 and the CSS of the returned framematches the CSS of the frame which was sent out. A downstream serviceloopback is similar to the upstream service loopback but facingdownstream from the ND. The upstream and downstream service loopback isgenerally associated to a provisioned rule.

Network loops occur when a physical loop is created in a network due tomisconnection of cables or misconfiguration of switches.

Loopbacks and network loops may cause a number of issues in a networkincluding loss of intended connectivity, interference with MAC addresslearning at switches, slowed traffic in the network because loopedframes are broadcast back into the network thus increasing traffic andslowing down the network, and broadcast storms.

Therefore, there is a need for detection of loopbacks and network loopsin a network, in order to locate the fault and remedy the problem in atimely manner without excessive disruption of the transmission of datain the network.

In one embodiment, a loopback detection beacon (LPDB) is used to detectloopbacks. The LPDB may be a data frame with specific protocol data unit(PDU) information that is used to identify the LPDB.

An exemplary embodiment to detect port loopback is shown in FIG. 6. LPDB711 has SA 711-1, DA 711-2 and PDU 711-4. The DA 711-2 may be an unusedMAC address. The LPDB is sent into the network via a port on anoriginating device, such as port 201-2 of ND 201.

In one embodiment, an LPDB is used to detect port loopback. This isshown in FIG. 7. The LPDB 711 is sent via a port such as port 201-2 on aND 201 and the originating port 201-2 is monitored. A loopback on port201-2 is detected if the LPDB 711 is returned to port 201-2 and matchesthe LPDB sent out. A test instance is set up and information relating tothe test instance is also stored within the ND, and a timer is set forthe test instance. The timer value is generally fixed and programmed,but any other techniques to set the timer for a given test instance canbe used. The PDU 711-4 includes information relating to the testinstance taking place. When the frame returns to the ND, the informationwithin PDU 711-4 is checked against the information relating to the testinstance stored within the ND. If the two sets of information match,then a loopback is detected. When the timer expires for a given testinstance, the information related to the test instance is removed. Ifthe LPDB returns to the ND later, there is no match found and statisticsare updated.

In a further embodiment, a timestamp associated with the LPDB is used todistinguish between hard cable loopback and remote facility loopback. Ifthe LPDB returns, the timestamp is compared to the time. If thedifference is smaller than a threshold (typically one transmissiondelay), then a hard cable loopback is detected, otherwise if thedifference is larger, a remote facility loopback is detected.

In another embodiment, the LPDB is used to detect tunnel loopback. Anexemplary LPDB 811 to detect a tunnel loopback is shown in FIG. 8 withSA 811-1, DA 811-2, PDU 811-4 and VLAN signature 811-5. In FIG. 9, LPDB811 is sent via a port 201-2 on ND 201 toward a Virtual Local AreaNetwork (VLAN) 903 associated with an 802.1Q standard tunnel. Thedeparting port 201-2 of the ND 201 is monitored and a tunnel loopback isdetected if the VLAN signature 811-5 of the returned LPDB matches thatof the departing LPDB 811 at the originating port 201-2. The monitoringcomprises storing information about the test instance, setting up atimer of a predetermined value and removing the information about thetest instance when a match is detected or the timer expires.

In another embodiment, the LPDB is used to detect service loopback. TheLPDB includes information that uniquely identifies the customer networkand is sent from a UNI in either an upstream direction or a downstreamdirection. An example LPDB 911 to detect service loopback is shown inFIG. 10. LPDB 911 comprises SA 911-1, DA 911-2, PDU 911-4 and VLANsignature 911-5. VLAN signature 911-5 further comprises uniqueidentification data, which includes a CSS 911-6 and NSS 911-7. Thedeparting port 201-2 of ND 201 is monitored. The monitoring comprisesstoring information about the test instance, setting up a timer of apredetermined value and removing the information about the test instancewhen a match is detected or the timer expires.

Upstream service loopback is detected if the CSS of the originatingframe matches the CSS of the returning frame. FIG. 11 shows thedetection of upstream service loopback. In FIG. 11, an LPDB 911 isgenerated at port 201-1 and sent via port 201-2 to the network. Rulesare sent by port 201-1 to port 201-2 to monitor the return of an LPDB911, with the CSS 911-6 of the returning frame 911 matching the CSS ofthe originating frame, a downstream service loopback is detected. Themonitoring includes storing information about the test instance andsetting a timer. The information stored about the test instance isremoved when a loopback is detected or the timer expires.

Similarly, downstream service loopback is detected if the CSS and NSS ofthe originating frame matches the CSS and NSS of the returning framewhen the LPDB is sent downstream from port 201-1 and returns to port201-1. In this case, the rules may be sent to port 201-2 in case theLPDB returns to that port which would indicate a misconfiguration andcan raise an alarm.

In one embodiment, the monitoring processes to detect loopbacks asdescribed above are implemented on the ND. In another embodiment, thesemonitoring processes are implemented on a separate device or server. Inthis case, the information about the test instance is stored on theseparate device, and if an LPDB is received, its information or the LPDBitself is forwarded to the separate device for processing. Informationabout several received LPDB can be aggregated into one message betweenthe ND and the separate device.

In one embodiment, the LPDB is generated by a customer using a softwaretool. In an additional embodiment, the tool is used to detect loopbacksand network loops; and determine the location of the loopbacks ornetwork loops. The software tool includes a programmable interface thatallows the user to configure the LPDB to their particular requirements,before the LPDB is generated. In one embodiment, this software tool isrun on the ND. In a further embodiment, this software tool is run on aseparate server or device.

In an embodiment, the LPDB is configured before generation depending onthe network to be tested or the detection needs, that is, whether it isbeing used to detect a port, tunnel or service loopback or the filtersand modifiers present in the rule information already in theconfiguration describing the forwarding path used for forwarding withinthe ND.

If a loopback is detected, then in one embodiment, affected traffic isdiscarded. In a further embodiment, future traffic is blocked at theport, by blocking the forwarding capabilities to and from the port. Forthe cases of port loopback, tunnel loopback and downstream serviceloopback, forwarding of traffic to the port is prevented. For the caseof upstream service loopback, forwarding of traffic from the port isprevented. In one embodiment, this blocking is revertive. This meansthat after the loopback is removed, the blocking is removedautomatically. In another embodiment, this blocking is non-revertive,that is, the operator has to remove the blocking manually.

In one embodiment, a network loop is detected using a network loopdetection beacon (NLDB).

An exemplary NLDB 1311 is shown in FIG. 12. This NLDB includes SA1311-1, DA 1311-2, PDU 1311-4 and VLAN signature 1311-5 . The VLANsignature 1311-5 includes CSS 1311-6 and NSS 1311-7.

In one embodiment, an NLDB is sent from a port of an originating device,such as an ND, into the network. FIG. 13 illustrates the NLDB 1311 beingsent from port 201-2 of ND 201, which is the originating device. Theport 201-2 is monitored. If the NLDB 1311 returns to ND 201 via adifferent port, such as port 201-1, the NLDB is forwarded to theoriginating port 201-2. If there is a match between the CSS 1311-6 andNSS 1311-7 of the returning NLDB and the CSS and NSS of the departingNLDB, then a network loop is detected. The monitoring comprises storinginformation about the test instance, setting up a timer of apredetermined value and removing the information about the test instancewhen a match is detected or the timer expires.

In one embodiment, the monitoring processes to detect network loops, asdescribed above, are implemented on the ND. In another embodiment, thesemonitoring process are implemented on a separate device or server.

In one embodiment, the NLDB is generated by a customer using a softwaretool. The tool may be used to detect loopbacks and network loops, and todetermine the location of the loopbacks or network loops. The softwareincludes a programmable interface that allows the user to configure theNLDB to the user's particular requirements, before the NLDB isgenerated. The software tool may be run on the ND, or on a separateserver or device.

The NLDB may be configured before generation based on the filters andmodifiers present in the rule information used for forwarding within theND.

If a network loop is detected on a port, traffic from that port may bediscarded. Alternatively, traffic may be blocked from entering the port.This blocking may be revertive (which means that after the cause of theloopback is removed, the blocking is removed automatically), ornon-revertive (which means the blocking has to be removed manually).

Alarms may be reported if a loopback or network loop is detected. Iftraffic is discarded, due to either loopbacks such as port or tunnelloopbacks being detected or a network loop being detected, then acritical alarm is reported.

The methods for testing for loopbacks can be generalized to amultiple-section network scenario, where different sections of thenetwork are assigned different levels, and domains are created based onthe levels. In one embodiment, each domain has a number, and the domainincludes one or more sections either at the same level as the domainnumber or the levels below. For example, a domain number 2 includes alllevel 2 sections and level 1 sections. A domain number 4 includes alllevel 4, level 3, level 2 and level 1 sections.

An exemplary multiple-section network 1200, with NDs 1211-1216 betweeneach section, is shown in FIG. 14. Each section has a different level,as shown in Table 1:

TABLE 1 Section and level corresponding to each section formultiple-section network 1200 Section Level 1201 6 1202 5 1203 4 1204 21205 4 1206 5 1207 6

The domain that each section belongs to is as shown in Table 2

TABLE 2 Domain to which each section belongs Domain Sections 6 1201,1202, 1203, 1204, 1205,1206, 1207 5 1202, 1203, 1204, 1205, 1206 4 1203,1204, 1205 2 1204

In one embodiment, domain numbers are assigned to LPDBs as part of theLPDB configuration process. Based on the assigned domain numbers, LPDBsare restricted to one or more selected sections to test for loops withinthose sections. The restriction is specified within a forwarding rule ateach ND at the interface to the next section. For example, withreference to the example above, an LPDB with a domain number 4 can onlytraverse sections 1203, 1204 and 1205, and is prevented, by a forwardingrule implemented within the NDs at the interfaces to section 1202 and1206, from entering sections 1202 and 1206, which are level 5.

In a further embodiment, an equal flag is added to the LPDB domainnumber as part of the domain number assigning process, so that LPDBs arenot forwarded outside sections with levels which do not have the samedomain number. Referring to the example above, an LPDB with an equalflag and domain number 4 remains in either section 1203 or 1205, andcannot enter section 1202, 1204 or 1206.

It should be understood that any two or more of the algorithms disclosedherein can be combined in any combination. Any of the methods,algorithms, implementations, or procedures described herein can includemachine-readable instructions for execution by: (a) a processor, (b) acontroller, and/or (c) any other suitable processing device. Anyalgorithm, software, or method disclosed herein can be embodied insoftware stored on a non-transitory tangible medium such as, forexample, a flash memory, a CD-ROM, a floppy disk, a hard drive, adigital versatile disk (DVD), or other memory devices, but persons ofordinary skill in the art will readily appreciate that the entirealgorithm and/or parts thereof could alternatively be executed by adevice other than a controller and/or embodied in firmware or dedicatedhardware in a well known manner (e.g., it may be implemented by anapplication specific integrated circuit (ASIC), a programmable logicdevice (PLD), a field programmable logic device (FPLD), discrete logic,etc.). Also, some or all of the machine-readable instructionsrepresented herein can be implemented manually as opposed toautomatically by a controller, processor, or similar computing device ormachine. Further, although specific algorithms are described withreference to flowcharts depicted herein, persons of ordinary skill inthe art will readily appreciate that many other methods of implementingthe example machine readable instructions may alternatively be used. Forexample, the order of execution of the blocks may be changed, and/orsome of the blocks described may be changed, eliminated, or combined.

It should be noted that the algorithms illustrated and discussed hereinas having various modules which perform particular functions andinteract with one another. It should be understood that these modulesare merely segregated based on their function for the sake ofdescription and represent computer hardware and/or executable softwarecode which is stored on a computer-readable medium for execution onappropriate computing hardware. The various functions of the differentmodules and units can be combined or segregated as hardware and/orsoftware stored on a non-transitory computer-readable medium as above asmodules in any manner, and can be used separately or in combination.

While particular implementations and applications of the presentdisclosure have been illustrated and described, it is to be understoodthat the present disclosure is not limited to the precise constructionand compositions disclosed herein and that various modifications,changes, and variations can be apparent from the foregoing descriptionswithout departing from the spirit and scope of an invention as definedin the appended claims.

What is claimed is:
 1. A method of determining the presence of aloopback in one or more networks, comprising: storing informationrelated to a test instance, wherein said stored information related tosaid test instance is in a protocol data unit (PDU) that includes aVirtual Local Area Network (VLAN) signature; sending a loopbackdetection beacon (LPDB) containing said stored information related tosaid test instance from a port on an originating device; monitoring saidport for a predetermined time period to detect LPDBs arriving at saidport during said predetermined time period; determining whether adetected LPDB contains information corresponding to said storedinformation related to said test instance, to detect the presence of aloopback; and determining that said detected loopback is a tunnelloopback if said detected LPDB contains information including said VLANsignature.
 2. The method of claim 1 in which said detected loopback is aport loopback or a service loopback, and wherein said method includesdetermining whether said detected loopback is a port loopback or aservice loopback.
 3. The method of claim 1 in which said originatingdevice is a network device.
 4. The method of claim 1 in which saidstored information related to said test instance is deleted if an LPDBarriving at said port and containing information corresponding to saidstored information is not detected within said predetermined timeperiod.
 5. The method of claim 1 which includes determining whether adetected port loopback is a hard cable loopback or a facility loopback.6. The method of claim 1 in which said information in said LPDB containsa time stamp, and the difference between said time stamp and the arrivaltime of a detected LPDB is used to determine whether said detectedloopback is a hard cable loopback or a remote facility loopback.
 7. Themethod of claim 6 in which said detected loopback is determined to be ahard cable loopback if said difference is smaller than a predeterminedthreshold, and is determined to be a facility loopback if saiddifference is greater than said predetermined threshold.
 8. The methodof claim 2 in which said LPDB includes a Virtual Local Area Network(VLAN) signature that includes a Customer Service Specification (CSS)and a Network Service Specification (NSS), and wherein said methodincludes determining whether said detected loopback is a serviceloopback if (1) the CSS of an LPDB arriving at said port matches the CSSof an LPDB sent from said port, or (2) the NSS of an LPDB arriving atsaid port matches the NSS of an LPDB sent from said port.
 9. The methodof claim 1 in which the monitoring and determining are implemented onsaid originating device.
 10. The method of claim 1 in which themonitoring and determining are implemented on a device or serverseparate from said originating device.
 11. The method of claim 1 inwhich affected traffic is discarded in response to the detection of aloopback.
 12. The method of claim 1 in which future traffic is blockedat said port in response to the detection of a loopback.
 13. The methodof claim 1 in which said LPDB is restricted to one or more selectedsections of the one or more networks.
 14. The method of claim 1 in whichsaid LPDB is generated using a software tool.
 15. The method of claim 1in which said software tool is used to detect loopback based on saidLPDB.
 16. The method of claim 1 in which said software tool is furtherused to configure said NLDB.
 17. The method of claim 12 in which saidblocking is revertive or non-revertive.
 18. A method of determining thepresence of a network loop, said method comprising: storing informationrelated to a test instance, wherein said stored information related tosaid test instance includes a Customer Service Specification (CSS) and aNetwork Service Specification (NSS); setting a time period related tothe test instance; sending a network loop detection beacon (NLDB)containing said stored information related to said test instance from aport on an originating device; monitoring said port for a predeterminedtime period to detect an NLDB arriving at said port during saidpredetermined time period; and determining whether a detected NLDBcontains information including said CSS and said NSS, to detect thepresence of a network loop.
 19. The method of claim 18 in which saidNLDB arrives at a second port on said originating device, and isforwarded to said port from which said NLDB was sent.
 20. The method ofclaim 18 in which traffic is either discarded or blocked if a networkloop is detected.
 21. The method of claim 20 in which said blocking isrevertive or non-revertive.
 22. The method of claim 18 in which an alarmis reported if a network loop is detected.
 23. The method of claim 18 inwhich said NLDB is generated using a software tool.
 24. The method ofclaim 23 in which said software tool is used to detect said networkloop.
 25. The method of claim 23 in which said software tool is used toconfigure said NLDB.
 26. The method of claim 18 in which saidoriginating device is a network device.